Astron. Astrophys. 324, L9-L12 (1997)

1. Introduction

The X-ray source 4U 1626-67 is a 7.7 s pulsar in a highly compact binary system of orbital period 2485 s. It is unusual in that it is one of the few low mass X-ray binary systems to contain an X-ray pulsar. While the X-ray emission is strongly modulated by the pulsar, there is no evidence for Doppler shifts induced by the orbital motion of the source, despite extensive searches. The implies that the projected orbital radius of the neutron star is small, i.e., sin i 13 m-lt-s (Levine et al. 1988). Optical pulsations were first detected by Ilovaisky et al. (1978) and interpreted as X-ray re-processing near to, or along, the line of sight to the X-ray source. Middleditch et al. (1981) found a single low frequency side-lobe which they interpret as arising from the optical re-processing of the primary X-rays on the companion star. Assuming the pulsar spins in the same sense as the orbital motion, these photons will be shifted to a lower frequency by the rotation frequency of the binary orbit. From the observed frequency shift of 0.4 mHz an orbital period of 2485 s and a projected semimajor axis of 0.4 lt-s is inferred. The current picture of 4U 1626-67 is of a highly compact system comprising a neutron star of mass 1 , with a 0.08 Main Sequence or 0.02 white dwarf companion (Verbunt et al. 1990).

For the first decade after its discovery 4U 1626-67 was rapidly spinning-up at a rate of  yr^-1. However, long term monitoring by the Burst and Transient Source Experiment (BATSE) on-board the Compton Gamma-ray Observatory beginning in 1991 April found that , and hence the accretion torque, had changed sign, resulting in a spin-down at nearly the same rate (Wilson et al. 1993). It is estimated that the reversal occurred in mid-1990. Observations of 4U 1626-67 during the earlier spin-up phase found that the phase averaged spectrum could be modeled by a blackbody of temperature, kT, 0.6 keV together with a power-law of photon index, , of 1 (e.g., Pravdo et al. 1979; Kii et al. 1986). In the 2-10 keV energy range the pulse profile consisted of a narrow pulse with a "notch", while at higher and lower energies this evolved into a roughly sinusoidal shape (Levine et al. 1988; Mavromatakis 1994). This strong energy dependence may result from anisotropic radiative transfer in a strongly magnetized plasma (Kii et al. 1986).

In addition to periodic pulsations, 4U 1626-67 also exhibits quasi periodic behavior. Both the X-ray and optical intensities show correlated flaring on timescales of 1000 s (Joss et al. 1978). The origin of this behavior is unknown. A 40 mHz quasi periodic oscillation (QPO) has been detected in X-rays (Shinoda et al. 1990) and more recently in the optical band (Chakrabarty et al. 1997).

Finally, the recent observation of an emission line complex near 1.0 keV by Angelini et al. (1995) is particularly interesting. This emission is interpreted as arising primarily from Ne K rather than from Fe L, based on the measured line energies and intensities. This result suggests that the companion star has evolved past its hydrogen burning stage. Ne is a by-product of He burning and therefore its overabundance suggests that the star is burning, or has burnt, He.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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